Modular prolinol chiral ligands for enantioselective addition of diethylzinc to aromatic aldehydes

Article abstract of DOI:10.1080/00397911.2011.615047

Enantioselective addition of diethylzinc (ZnEt2) to a series of aromatic aldehydes was promoted using a modular prolinol chiral ligand, 2-(S)-2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)-4-nitrophenol (1a), without using titanium complex. The catalytic system employing 15 mol% of 1a was found to promote the addition of diethylzinc to a wide range of aromatic aldehydes with electron-donating and electron-withdrawing substituents, giving up to 86% ee of the corresponding secondary alcohol under mild conditions.

Full text of DOI:10.1080/00397911.2011.615047

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Modular Prolinol Chiral Ligands for
Enantioselective Addition of Diethylzinc
to Aromatic Aldehydes
Shaohua Gou a b , Zhongbin Ye a b , Mingming Feng b & Wenchao
Jiang b
a State Key Laboratory of Oil and Gas Reservoir Geology and
Exploitation, Southwest Petroleum University, Chengdu, China
b School of Chemistry and Chemical Engineering, Southwest
Petroleum University, Chengdu, China
Accepted author version posted online: 22 Feb 2012.Version of
record first published: 07 Jan 2013.
To cite this article: Shaohua Gou , Zhongbin Ye , Mingming Feng & Wenchao Jiang (2013): Modular
Prolinol Chiral Ligands for Enantioselective Addition of Diethylzinc to Aromatic Aldehydes, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:7, 941-950
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Synthetic Communications1, 43: 941–950, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.615047
MODULAR PROLINOL CHIRAL LIGANDS FOR
ENANTIOSELECTIVE ADDITION OF DIETHYLZINC TO
AROMATIC ALDEHYDES
Shaohua Gou,1,2 Zhongbin Ye,1,2 Mingming Feng,2 and
Wenchao Jiang2
1State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,
Southwest Petroleum University, Chengdu, China
2School of Chemistry and Chemical Engineering, Southwest Petroleum
University, Chengdu, China
GRAPHICAL ABSTRACT
Abstract Enantioselective addition of diethylzinc (ZnEt2) to a series of aromatic aldehydes
was promoted using a modular prolinol chiral ligand, 2-(S)-2-(hydroxydiphenylmethyl)
pyrrolidin-1-yl)methyl)-4-nitrophenol (1a), without using titanium complex. The catalytic
system employing 15 mol% of 1a was found to promote the addition of diethylzinc to a wide
range of aromatic aldehydes with electron-donating and electron-withdrawing substituents,
giving up to 86% ee of the corresponding secondary alcohol under mild conditions.
Keywords Addition reaction; aldehyde; C1-symmetric; diethylzinc; prolinol
INTRODUCTION
Enantioselective addition of diethylzinc (Et2Zn) to aldehydes is a fundamental
and most successful C-C bond-formation reaction in the field of organic chemistry,
natural products, and biologically active compounds.[1] It has received widespread
attention because it has been one of the most efficient methods for generating opti-
cally active secondary alcohols since 1980s.[1,2] The importance of this structural
Received June 16, 2011.
Address correspondence to Shaohua Gou, School of Chemistry and Chemical Engineering,
Southwest Petroleum University, Chengdu 610500, China. E-mail: shaohuagou@swpu.edu.cn
941

942
S. GOU ET AL.
Figure 1. Reported C2-symmetric auxiliary’s ligand.
feature arises mainly from the fact that it is part of many natural products or pre-
cursors as an important synthetic intermediate.[1] For this purpose, a wide variety
of chiral catalysts, such as amino alcohols,[2] diamines,[3] aminothiols,[4] aminodise-
lenides,[5] aminodisulfides,[6] diols,[7] 3,30-diphsphoryl-1,10-bi-naphthols,[8] and phos-
phoramides[9] have been synthesized and applied for the addition of diethylzinc to
aldehydes. Despite the achievements made in this field of the addition of aldehydes,
developing highly efficient catalyst systems and meeting practical application have
been continuously attractive subjects for organic chemists. Among the chiral ligands
reported, b-amino alcohols are the most often used chiral auxiliaries.[1,2] Recently,
several C2-symmetrical auxiliaries based on prolinol (5a) have been synthesized by
Trost and Chan groups and applied in the catalytic enantioselective addition reac-
tion (Fig. 1).[10] However, to our knowledge, few C1-symmetric prolinol ligands
are studied in the addition of diethylzinc to aldehyde, which derive from substituted
phenol. In this article, we report the details of the synthesis of a new optically active
C1-symmetric L-prolinol ligand (Scheme 1) and their catalyzed asymmetric addition
reaction of diethylzinc to aromatic aldehydes.
RESULTS AND DISCUSSION
Catalyst System Screening
In the preliminary studies, we investigated the addition reaction of diethylzinc
(Et2Zn) to benzaldehyde (2a) in the presence of 15 mol% chiral ligand (1a) in varying
solvent at 0 ꢀC under nitrogen atmosphere (Table 1, entries 1–11). It was found that
solvent drastically affected the enantiomeric excess (ee) of this asymmetric reaction.
In contrast to the general results that hexane can give good results, a hexane solution
of benzaldehyde reacting with 1.5 equiv. of diethylzinc (Et2Zn, in hexane solution) in
Scheme 1. Modular proline C1-symmetric chiral ligands.

ENANTIOSELECTIVE ADDITION OF DIETHYLZINC
943
Table 1. Effect of 1a for the enantioselective addition of diethylzinc to benzaldehyde
Entrya
Solvent
Time (h)
Yield (%)b
Ee (%)c
1
2
Hexane
Toluene
30
24
24
24
24
24
24
24
24
24
30
24
12
78
68
57
48
59
57
30
26
10
16
77
56(S)
74(S)
58(S)
55(S)
54(S)
52(S)
53(S)
60(S)
56(S)
51(S)
23(S)
86(S)
3
Toluene=hexane (v=v, 1:1)
Toluene=hexane (v=v, 1:2)
Toluene=hexane (v=v, 1:3)
Xylene
4
5
6
7
Xylene=hexane (v=v, 1:1)
THF
8
9
THF=hexane (v=v, 1:1)
Et2O
CH2Cl2
10
11
12d
Toluene
aConditions: concentration of 2a, 0.25 M; Et2Zn: 1.5 equiv. in hexane solution.
bIsolated yields.
cThe ee was determined with a chiral GC G-TA column, and the (S or R)-configuration was confirmed
by comparison with the reported configuration.[11a,b]
dEt2Zn: 1.5 equiv. in PhCH3. For details, see the Experimental section.
the presence of 15 mol% of chiral ligand 2a at 0 ꢀC can only give (S)-1-phenyl-1-pro-
panol in 12% isolated yield with 56% ee (Table 1, entry 1). However, when the same
reaction was employed in toluene under otherwise identical conditions, both the iso-
lated yield and ee of the reaction product were considerably improved to 78% and
65%, respectively (Table 1, entry 2). The isolated yields and ee’s of the reactions
decreased with an increase of hexane content when a mixed solvent of
toluene-hexane was used (Table 1, entries 3–5). It is might be arritibuted to the chiral
ligand 1a can be dissolved in toluene easily and dissolved in hexane slightly. The
same phenomena were also observed when xylene-hexane was used as solvent
(Table 1, entries 6–7). For THF, Et2O and THF-hexane as solvent, because of their
coordination to diethylzinc, low yields and moderate enantioselectivities were
observed (Table 1, entries 8–10). However, for dichloromethane (CH2Cl2) as the
solvent, poor yield and enantioselectivity were obtained (Table 1, entry 11). Gratify-
ingly, a significant improvement (86% ee, 77% yield) was achieved when the hexane
solvent was evaporated with the addition of diethylzinc to benzaldehyde (Table 1,
entry 12; for details see the experimental section).
Next, we investigated the varying loadings of 1a and Et2Zn, optimum tempera-
ture, and concentration of 2a using a 1a catalyst system (Table 2, entries 1–10). It
was found that better enantioselectivity could not be obtained when lowering or
increasing the loading of 1a (Table 2, entries 2–4), although better yields could be
obtained when increasing the loading of 1a from 15 to 20 mol% (Table 2, entry 4).

944
S. GOU ET AL.
Table 2. Optimization of the catalytic system for the enantioselective addition of diethylzinc to benzal-
dehyde catalyzed by 1a
Entrya 1a (mol%) Concentration of 2a Et2Zn (equiv.) Temp. (ꢀC) Time (h) Yield (%)b Ee (%)c
1
2
15
10
5
0.25
0.25
0.25
0.25
0.25
0.25
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.0
1.2
0
0
24
30
48
20
40
10
10
40
20
40
77
70
46
83
75
96
91
69
91
65
86(S)
71(S)
53(S)
78(S)
85(S)
58(S)
74(S)
73(S)
85(S)
73(S)
3
0
4
20
15
15
15
15
15
15
0
5
ꢁ20
20
0
6
7
8
0.15
0.25
0.25
0
9
10
0
0
aConditions: solvent: PhCH3; Et2Zn–hexane solution, similar to Table 1, entry 12.
bIsolated yields.
cThe ee was determined with a chiral GC G-TA column, and the (S)-configuration was confirmed by
comparison with the reported configuration.[11a,b]
Changing the reaction temperature (20 or ꢁ20 ꢀC) could also not improve the
enantioselectivity (Table 2, entries 5 and 6). Similarly, changing the concentration of
2a and loading of Et2Zn could not also improve the enantioselectivity (Table 2,
entries 7–10). Herein, the ZnEt2 may act as a Lewis acid and 1a may act as a Lewis
base in the catalyst system; increasing or decreasing the loading of 1a or ZnEt2 could
not afford a best pKa value for the enantioselective addition of diethylzinc to alde-
hydes.[2e] The optimal catalyst system and reaction conditions were 15 mol% 1a, 1.5
equiv. Et2Zn, and 0.25 M of 2a in PhCH3 at 0 ꢀC.
Substrate Generality
To study the generality of the 1a catalyst system for the enantioselective
addition of diethylzinc to various aromatic aldehydes, a number of aromatic
aldehydes having electron-donating and electron-withdrawing groups, a- and
b-naphthaldehydes, and (E)-cinnamaldehyde were examined under the optimized
conditions summarized in Table 3. In comparison to the results obtained with 2a,
the poor electron-donating substituents, 4-Methyl group, gave similar results to 2a
(Table 3, entry 2 vs 1). However, the 2-methyl group led to a decrease in the ee
and yield of the products 3c after a longer time (Table 3, entry 3 vs entry 1). In
the case of stronger electron-donating substituents, MeO group, lower yields with
moderate ee (Table 3, entries 4 and 5) were obtained. Electron-withdrawing groups
(F, Cl, Br, I, and CF3, Table 4, entries 6–11) showed variation in the yields (67–81%)
and ees (53–82%). In particular, the 4-(trifluoromethyl)benzaldehyde gave only 53%
ee and yields (Table 3, entry 11). (E)-Cinnamaldehyde only gave 47% yield with 49%
ee (Table 4, entry 12). Reaction of a- and b-naphthaldehydes 2j and 2k resulted with
up to 83% ee and 72% ee, respectively (Table 4, entries 13 and 14). In general, mod-
erately to good yields and enantioselectivities of the secondary alcohols 3a–n were
obtained (Table 4); electron-donating and bigger substituted aromatic aldehydes
gave less favorable results than electron-withdrawing substituted aromatic

ENANTIOSELECTIVE ADDITION OF DIETHYLZINC
945
Table 3. Enantioselective addition of diethylzinc to various aromatic aldehydes catalyzed by 1g
Entrya
Aldehyde
Benzaldehyde (2a)
Time (h)
Yield (%)b
Ee (%)c
1
2
24
24
30
48
48
30
30
40
40
40
48
48
24
30
77
79
70
68
43
81
78
69
72
78
67
47
88
77
86(S)
84(S)
68(S)
60(S)
59(S)d
82(S)
78(S)
67(S)
71(S)
85(S)
53(S)e
49(S)d
83(S)d
72(S)d
4-Methyl-benzaldehyde (2b)
2-Methyl-benzaldehyde (2c)
4-Methoxy-benzaldehyde (2d)
3-Methoxy-benzaldehyde (2e)
4-Fluoro-benzaldehyde (2f)
4-Chloro-benzaldehyde (2g)
2-Chloro-benzaldehyde (2h)
4-Bromobenzaldehyde (2i)
3
4
5
6
7
8
9
10
11
12
13
14
4-Iodobenzaldehyde (2j)
4-(Trifluoromethyl)benzaldehyde (2k)
(E)-Cinnamaldehyde (2l)
Naphthalene-2-carbaldehyde (2m)
Naphthalene-1-carbaldehyde (2n)
aConditions: solvent: PhCH3; 0 ꢀC; concentration of 2: 0.25 M; Et2Zn in hexane solution. The procedure
is the same as in Table 1, entry 12.
bIsolated yields.
cThe ee was determined with a chiral GC G-TA column, and the (S)-configuration was confirmed by
comparison with the reported configuration.[11a–c]
dThe ee was determined using a chiral OD-H or OD column.[8a–d]
eThe ee was determined using a chiral OJ-H column.[11f–g]
aldehydes. These results revealed that the 1a catalyst system was effective for the
1,2-addition of diethylzinc to various aromatic aldehydes.
Catalytic Mechanism Considerations
According to the previous works in the field of the enantioselective addition
diethylzinc to aromatic aldehydes,[1–12] herein the Zn(II) complex might play a multi-
functional role in this reaction. The metal moiety of the Zn(II) catalyst system act as
a Lewis acid to activate the aldehyde and engender one species. On the other hand,
the tertiary amine and OH groups of the catalyst system might act as a Lewis base to
activate the ZnEt2 and engender another species (Et-). Then the activated ZnEt2
transfers the species (Et-) to the activated aldehyde and affords the corresponding
product.
CONCLUSION
In summary, the chiral ligand 1a, which derives from L-proline and was readily
prepared in several steps from commercially available starting materials, showed

946
S. GOU ET AL.
good catalytic activities and moderate enantioselectivities (up to 86% ee) in the asym-
metric additions of diethylzinc to various aromatic aldehydes. Further investigation
on the applications of these ligands for other asymmetric reactions is ongoing.
EXPERIMENTAL
All reactions were conducted in oven-dried glassware under an inert atmos-
phere of nitrogen with anhydrous solvents unless otherwise stated. The solvents were
purified and dried according to standard procedures. Analytical thin-layer chroma-
tography (TLC) was performed on alumina- or glass-backed silica plates (F254,
250-micron thickness) and visualized with untraviolet (UV) light. Flash column
chromatography was carried out on silica gel 60 (250–400 mesh) under air pressure.
Enantiomeric ratios of the products were determined using chiral gas chromatography
(GC) and high-performance liquid chromatography (HPLC) techniques. Specific
rotations were determined as [a]D22 (c ¼ 0.5 g=mL in CH2Cl2). Melting points are
uncorrected. 1H NMR (300 MHz) and 13C NMR (75 MHz) chemical shifts in CDCl3
are quoted as d values relative to tetramethylsilane (TMS) (d ¼ 0.00) and CDCl3
(d ¼ 77.0), respectively, in parts per million (ppm), and coupling constants are in hertz
(Hz). The following abbreviations are used to indicate the multiplicity: s, singlet; d,
doublet; t, triplet; q, quartet; and m, multiplet. High-resolution mass spectra (HRMS)
were obtained using HPLC-Mass electrospray ionization (ESI) mode.
Materials
S-Proline (4a), 2-(bromomethyl)-4-nitrophenol 6a, diethylzinc (Et2Zn), and all
aldehydes were commercially available and used without further purification unless
otherwise noted. (S)-Prolinol (5a) was synthesized according to the literatures
procedure.[13]
General Procedure for the Synthesis of Chiral Ligand 1a (see
Scheme 1)
2-(Bromomethyl)-4-nitrophenol 6a (0.3 g, 1.3 mmol) was added in one portion
to a stirred and cooled solution of (S)-prolinol 5a (1.32 g, 5.21 mmol) and K2CO3

ENANTIOSELECTIVE ADDITION OF DIETHYLZINC
947
(2.88 g, 20.84 mmol) in dry dimethylformamide (DMF, 10 mL). The ice bath was
removed after the addition, and the resulting solution was allowed to stir at room
temperature (about 25 ꢀC) for 24 h before it was diluted with water (100 mL) and
ethyl acetate (EA, 100 mL). The two phrases were separated, and the aqueous phase
was extracted with EA (3 ꢂ 20 mL). The combined organic phases were washed with
water (2 ꢂ 20 mL) and brine, dried (Na2SO4), and evaporated. The crude products
were purified using column chromatography on silica gel (EA–hexane); the corre-
sponding products were obtained as yellow solid in 71% yield, mp 85–87 ꢀC,
[a]D20 ¼ þ 25.90 (CH2Cl2, c ¼ 0.5 g=mL). H NMR (300 MHz, CDCl3) d (ppm) 7.95
1
(d, J ¼ 2.4 Hz, 1H, Ar-H, C8-H), 7.92 (d, J ¼ 2.7 Hz, 1H, Ar-H, C10-H), 7.08–7.68
(m, 10H, Ar-H, C11-C15-H), 6.69 (d, 1H, J ¼ 9.0 Hz, ArH, C7-H), 4.95 (s, 2H,
2OH), 3.52 (s, 2H, -N-CH2-), 2.11 (m, 2H, -CH2CH2N-), 1.61–1.64 (m, 4H,
-CH2CH2CH2N-), 1.25–1.27 (m, 1H, Ph2CCHOH). 13C NMR (75 MHz, CDCl3) d
(ppm) 159.9 (C16), 145.7 (C15), 140.1 (C14), 128.6 (C13), 128.5 (C12), 127.3
(C11), 126.1 (C10), 125.9 (C9), 121.8 (C8), 116.3 (C7), 80.1 (C6), 72.1 (C5), 44.1
(C4), 35.1 (C3), 29.3 (C2), 24.1 (C1) ppm. HRMS (ESI, CH3OH): calcd. for
(Mþ þ 1) for C24H25N2O4: exact mass: 405.1814, molecular weight: 405.4663, found:
405.1800. Anal. for C24H24N2O4: C, 71.27%, found: 71.42%; H, 5.98%, found:
6.11%; and N%: 6.93%, found: 7.11%.
Typical Procedure for the Catalytic Addition of Diethylzinc to
Aromatic Aldehydes
A solution ofdiethylzinc (1.0 M in hexane, 0.375 mL, 0.375 mmol) was added to
a solution of 1a (10.1 mg, 0.025 mmol) in PhCH3 (1.0 mL) under a nitrogen atmos-
phere at 0 ꢀC, and the reaction mixture was stirred for 30 min at room temperature
(about 25 ꢀC). Then, 1.0 mL dry PhCH3 was added after the mixture completely eva-
porated under vacuum evaporation. The reaction mixture was then cooled to 0 ꢀC,
and the corresponding aromatic aldehyde (0.25 mmol) was added. Stirring continued
for the stated times. The reaction mixture was quenched with HCl (1.0 M, 2.0 mL) at
0 ꢀC, and the product was extracted with (3 ꢂ 5 mL) ethyl acetate. The combined ethyl
acetate extracts were dried over Na2SO4 and evaporated to dryness under vacuum
pressure. The residue was purified by silica-gel column chromatography (hexane=
ethyl acetate, 10=1, v=v) to afford the secondary alcohol products. The enantioselec-
tivities of the reactions were determined by Chiral GC G-TA, OJ-H, or OD-H col-
umns. Compounds 3a–n are known compounds; they were characterized by
comparing their 1H and 13C NMR spectra with those published in the litera-
ture.[10a–h]
Selected Data
(S)-1-Phenyl-propan-1-ol (3a)[8c,11a–c]. Enantiomeric excess was determined
on a Chiral GC G-TA column (100 ꢀC, 2.0 mL=min, tR ¼ 9.0 min, tR ¼ 9.2 min).
(S)-1-p-Tolyl-propan-1-ol (3b)[8c,11a–c]. Enantiomeric excess was determined
on a Chiral GC G-TA column (115 ꢀC, 2.0 mL=min, tR ¼ 13.7 min, tR ¼ 13.4 min).

948
S. GOU ET AL.
(S)-1-o-Tolyl-propan-1-ol (3c)[8c,11a–c]. Enantiomeric excess was determined
on a Chiral GC G-TA column (115 ꢀC, 2.0 mL=min, tR ¼ 14.2 min, tR ¼ 12.7 min).
(S)-1-(4-Methoxy-phenyl)-propan-1-ol (3d)[8c,11a–c]. Enantiomeric excess
was determined on a Chiral GC G-TA column (110 ꢀC, 2.0 mL=min, tR ¼ 41.1 min,
min, tR ¼ 39.7 min).
(S)-1-(3-Methoxy-phenyl)-propan-1-ol (3e)[11a–d]. Enantiomeric excess was
determined on a Chiral HPLC OD (UV detector, 254 nm, hexane= i-PrOH ¼ 9=1,
1.0 mL=min, tR ¼ 10.8 min, tR ¼ 10.0 min).
(S)-1-(4-Fluoro-phenyl)-propan-1-ol (3f)[11a–c]. Enantiomeric excess was
determined on a Chiral GC G-TA column (110 ꢀC, 2.0 mL=min, tR ¼ 10.9 min,
tR ¼ 10.1 min).
(S)-1-(4-Chloro-phenyl)-propan-1-ol (3g)[8c,11a–c]. Enantiomeric excess was
determined on a Chiral GC G-TA column (135 ꢀC, 3.0 mL=min, tR ¼ 7.5 min,
tR ¼ 8.3 min).
(S)-1-(2-Chloro-phenyl)-propan-1-ol (3h)[8c,11a–c]
Enantiomeric excess was determined on a Chiral GC G-TA column (135 ꢀC,
3.0 mL=min, tR ¼ 11.2 min, tR ¼ 11.7 min).
(S)-1-(4-Bromo-phenyl)-propan-1-ol (3i)[8c,11a–c]
Enantiomeric excess was determined on a Chiral GC G-TA column (130 ꢀC,
3.0 mL=min, tR ¼ 15.9 min, tR ¼ 15.2 min).
(S)-1-(4-Iodo-phenyl)-propan-1-ol (3j)[11a–c]
Enantiomeric excess was determined on a Chiral GC G-TA column (130 ꢀC,
3.0 mL=min, tR ¼ 29.5 min, tR ¼ 28.0 min).
(S)-1-(4-Trifluoromethyl-phenyl)-propan-1-ol (3k)[8c,11a–h]
Enantiomeric excess was determined on a Chiral HPLC OJ-H (UV detector,
254 nm, hexane=i-PrOH ¼ 98=2, 1.0 mL=min, tR ¼ 17.9 min, tR ¼ 16.4 min).
(S)-(E)-1-Phenyl-pent-1-en-3-ol (3l)[8c,11a–h]
Enantiomeric excess was determined on a Chiral HPLC OD-H (UV detector,
254 nm, hexane=i-PrOH ¼ 9=1, 1.0 mL=min, tR ¼ 13.5 min, tR ¼ 9.1 min).
(S)-2-Naphthalen-2-yl-propan-1-ol (3m)[8c,11a–h]
Enantiomeric excess was determined on a Chiral HPLC OD-H (UV detector,
254 nm, hexane=i-PrOH ¼ 9=1, 1.0 mL=min, tR ¼ 10.1 min, tR ¼ 9.4 min).

ENANTIOSELECTIVE ADDITION OF DIETHYLZINC
949
(S)-1-Naphthalen-2-yl-propan-1-ol (3n)[8c,11a–h]
Enantiomeric excess was determined on a Chiralcel HPLC OD column (UV
detector, 254 nm, 4=96 i-PrOH=hexane, 0.5 mL=min tR ¼ 31 min, tR ¼ 27 min).
ACKNOWLEDGMENT
The authors gratefully acknowledge the Open Fund (No. PLN1105) of Key
Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest
Petroleum University) for financial support.
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